This invention relates generally to wireless data communication systems. More particularly, the invention relates to mobile data base stations supporting transmission and reception of data in multiple modes.
The modern analog cellular system for mobile wireless duplex voice transmission is called xe2x80x9cAdvanced Mobile Phone Servicexe2x80x9d (AMPS). The AMPS cellular network uses the FCC assigned carrier frequency range of 800 to 900 MHz. Automobile-mounted cellular units transmit voice signals to a cellular base station within a given cell using up to one watt of power. Hand-held cellular units using battery power supplies transmit voice signals to a cellular base station within a given cell using up to one quarter watt of transmission power.
The analog human voice was the signal that the AMPS system was first designed to communicate. The AMPS system has been optimized for carrying as many analog voice signals within a given bandwidth of a channel as possible. Mobility of the cellular telephone using low power mobile units, FM modulation, and the higher carrier frequency range (800 MHz-900 MHz) is achieved through a cellular arrangement of base stations whereby a user""s signal is handed off to the next cell site as he or she moves into a different cell area. This cellular hand-off can cause a temporary loss in transmission or reception. Temporarily losing a voice signal is not critical because a user knows when there is a signal loss and the voice information can be retransmitted. However, signal loss, even though temporary, poses special problems for transmission of digital data. Some other AMPS cellular problems causing loss in voice signals are drops in signal strength, reflections, Rayleigh fading, and cellular dead spots.
The availability of portable computers naturally results in the desire to conduct wireless transmission of digital data from a remote location. Presently, the AMPS voice cellular system is being used to transmit digital data in the form of circuit-switched cellular data across AMPS carrier channels. Raw (baseband) digital data must be converted so that it can be transmitted and received across the analog AMPS system. One disadvantage to using the AMPS system for data transmission is that a narrow channel bandwidth and errors in transmission limit the baud rate for transmitting and receiving the digital data. As previously stated, loss of raw digital data may be caused by other problems in the AMPS mobile cellular system.
In the circuit-switched mode of data communication, a single channel is set aside for specific communication, and is dedicated thereto until that communication is complete. When sending data in a conventional cellular circuit-switched mode, a modem creates a waveform from the data in a manner similar to that of a wireline modem. The modem combines amplitude and phase modulation to create modulation based upon multiple bits per symbol. The resulting modulated signal is then transmitted over the cellular voice channel in the same way that it would be sent over a wireline connection, i.e., as an audio signal. The modulation constellations used for cellular modems are, although similar, generally simpler than that used for their wireline counterparts. However, they are somewhat similar. Consequently, the modulated circuit-switched cellular signals are particularly vulnerable to the sort of degradations that occur on a normal cellular voice channel which is far noisier than a typical wireline voice channel.
In general, performance of cellular circuit-switched modems is quite poor as soon as the signal strength becomes relatively low or the interference level becomes high. For example, while the sensitivity of the cellular voice receiver is about xe2x88x92103 dBm in a fading channel, the circuit-switched connection begins to degrade at about xe2x88x9280 dBm. Normally, such degradation can be overcome by shifting the bit rate downwards. However, even at 4.8 kbps, it is difficult to maintain a reliable connection when the signal level dips below xe2x88x9290 dBm. Even at high signal levels in the cellular circuit-switched arrangement, it is rare to be able to achieve more than 14.4 kbps data rate.
In addition, the performance of the circuit-switched modem is impacted by the nature of the backhaul as well as the nature of the airlink. The backhaul can cause severe echo problems that necessitate good echo cancellation techniques requiring complex circuitry resulting in increased costs and additional station space. Thus, conventional circuit-switched AMPS data transmission is expensive as well as risky.
Another problem of data transmission with a mobile subscriber station occurs when a subscriber station is moving at the edge of a cell or coverage area. Such circumstances can result in a substantial loss of data, or substantial delays due to the necessity of re-transmitting the data. These problems have been addressed in part by the Cellular Digital Packet Data (CDPD) system described in the CDPD specification, Version 1.1, incorporated herein as background information. The CDPD communication system shares the same carrier frequencies assigned to the AMPS channels as indicated in the CDPD specification.
The base unit or mobile data base station (MDBS 1, as illustrated in FIG. 1), of an exemplary CDPD system utilizes a channel within an AMPS cell to establish a link and communicate to a user""s mobile end system. The MDBS may use other frequencies outside of AMPS that are made available to it by service providers. The mobile end system or subscriber station (M-ES 2) is a portable computer, hand-set or other portable electronic device containing a subscriber communication unit. The MDBS serves as a communication link between the user of the M-ES 2 and a service provider""s network of wire lines, microwave links, satellite links, AMPS cellular links, or other CDPD links (such as mobile data intermediate system MD-IS 3, intermediate systems 4, 5, 6) to convey data to another mobile end system, computer network, or non-mobile or fixed end-user system (F-ES 7, 8).
The CDPD network is designed to operate as an extension of existing communication networks, such as AMPS networks and the Internet network. From the mobile subscriber""s perspective, the CDPD network is simply a wireless mobile extension of traditional networks. The CDPD network shares the transmission facilities of existing AMPS networks and provides a non-intrusive, packet-switched data service that does not impact AMPS service. In effect, the CDPD network is entirely transparent to the AMPS network, which is xe2x80x9cunawarexe2x80x9d of the CDPD function.
The CDPD system employs connectionless network services (CLNS) in which the network routes each data packet individually based on the destination address carried in the packet and knowledge of current network topology. The packetized nature of the data transmissions from an M-ES 2 allows many CDPD users to share a common channel, accessing the channel only when they have data to send and otherwise leaving it available to other CDPD users. The multiple access nature of the system makes it possible to provide substantial CDPD coverage to many users simultaneously with the installation of only one CDPD transmitter/receiver in a particular sector (transmitting range and area of a standard AMPS base station transceiver).
The airlink interface portion of the CDPD network consists of a set of cells. A cell is defined by the geographical boundaries within which the RF transmission between an M-ES and an MDBS are received at acceptable levels of signal strength. It is not sufficient that the subscriber receive an adequate signal level. The base station must also receive a good signal from the subscriber station (M-ES) for the subscriber to be considered within the cell. The transmitter supporting the cell may be located centrally within the cell, with transmission being carried out via an omni-directional antenna, or the transmitter located at the edge of a cell and transmission carried out via a directional antenna. This second type of cell is also referred to as a sector. In typical configurations, the transmitters for several sectors are co-located. The area served by a set of cells has some area overlap, so that a moving mobile end system can maintain continuous service by switching from one cell to an adjacent cell in a manner roughly analogous to the standard hand-off in an AMPS system. The two cells are considered to be adjacent if an M-ES can maintain continuous service by switching from one cell to the other. This switching process, called cell transfer, is done independently of normal AMPS hand-off procedures.
In FIG. 1, the interface (A) between the mobile end system 2 and the MDBS 1 is an xe2x80x9cair interfacexe2x80x9d constituted by a radio frequency link using standard AMPS frequencies. The MDBS 1 is connected to other mobile data base stations through various public and private data networks. One example is mobile data intermediate system (MD-IS) 3. A number of mobile data base stations can be under the control of a single mobile data intermediate system. The mobile data intermediate systems are connected to each other through intermediate systems such as 4 and 5 in FIG. 1.
The intermediate systems are constituted by at least one node connected to more than one sub-network (such as intermediate system MD-IS 3). The intermediate system has a primary role of forwarding data from one sub-network to another. The mobile data MD-IS 3 performs data packet routing based on knowledge of the current location of each mobile end system within the range of the mobile data base stations under the control of the MD-IS. The MD-IS is usually the only network entity that is xe2x80x9cawarexe2x80x9d of the location of any of the mobile end systems. However, under some circumstances (as defined by the CDPD specification) particular mobile database stations will keep track of the behavior of specific subscriber stations. A CDPD-specific Mobile Network Location Protocol (MNLP) is operated between each MD-IS (through the intermediate system) to exchange location information regarding the mobile end systems.
The overall CDPD network is controlled by a network management system (NMS) 10 having an interface with at least one mobile data intermediate system 3. Using a special protocol, programming instructions can be transmitted from the NMS 10 through the MD-IS 3 to any number of mobile data base stations under the proper conditions.
Such programming instructions can be used to convey useful network data to the MDBS, as well as configuring the operation of an MDBS with respect to such critical features as maintaining channel queues. The NMS also controls other CDPD system characteristics such as the timing of paging messages to coincide with the non-dormant periods of the M-ES hand-sets. The functions and protocol as carried out by each of the mobile end systems and the mobile data base station are explained in greater detail in the CDPD specification, Parts 402 and 403.
FIG. 2 provides a comparison between the CDPD network illustrated in FIG. 1 and the standard AMPS network. The MDBS 1 is the CDPD equivalent to an AMPS base station 21. Both serve as links to mobile users, 2, 2xe2x80x2, and 2xe2x80x3 for the CDPD system and 22, 22xe2x80x2 and 22xe2x80x3 for AMPS users. Also, the MDBS 1 is preferably located with the AMPS base station 21.
The MD-IS 3 which acts as a local controller for the CDPD mobile data base stations connected thereto is equivalent to the mobile telephone switch office (MTSO) 23 used to control a plurality of AMPS base stations 21, 21xe2x80x2 and 21xe2x80x3. In the AMPS system, the MTSO 23 can be connected to the various base stations 21, 21xe2x80x2, 21xe2x80x3 by way of communication links, either over dedicated landlines or through a Public Switched Telephone Network (PSTN). Likewise, the connection between MD-IS 3 and the various mobile data base stations 1, 1xe2x80x2, 1xe2x80x3 controlled thereby is made in the same manner. However, different signaling protocols are used than those found in the AMPS system.
In comparison to AMPS, the infra-structure requirements of CDPD are very small. The CDPD base station equipment is located (preferably) at a cellular carrier""s cell site along side existing AMPS base station cellular equipment. The multiple access nature of the CDPD system makes it possible to provide substantial CDPD coverage to many users simultaneously with the installation of only one CDPD radio in a given sector. This multiple access is the result of a mobile end-system accessing the CDPD channel only when there is data to be sent.
The AMPS base station and the MDBS can use the same RF equipment if both are co-located. By contrast, the MTSO of the AMPS system and the MD-IS of the CDPD system do not have to be co-located in order to share RF links. In the AMPS system, the MTSO 23 has the responsibility of connecting the AMPS base station and the mobile station to another party 28 through a PSTN 24. The intermediate system 4 of the CDPD roughly corresponds to the use of the PSTN by the AMPS system. Like the AMPS system, the CDPD system must also use the public switch telephone network or another landline network for completing calls to remote parties or systems via a phone system terminal network 28. However, the CDPD system employs a different protocol than that used by the AMPS system for completing calls over a PSTN.
In general, the CDPD system illustrated in FIG. 1 operates to provide service to manage data communications to subscribers over a wide geographic range. When a mobile end system or subscriber station is located, data packets are routed directly to and from it by the MD-IS via the MDBS. The route via which data packets are forwarded to and from a mobile end system or subscriber station changes when the mobile end system moves.
The MDBS maintains zero or more (up to the MDBS transmission capability) channel streams across the airlink interface, as directed by the MD-IS controlling that MDBS. The MDBS instructs all subscriber units to change channels when necessary such as when an AMPS communication is detected on the CDPD channel. Each subscriber station""s terminal stream is carried on one channel stream at a time, normally selected by the mobile subscriber, preferably based upon data received from the MDBS regarding optimum channels for CDPD use. The forward and reverse traffic in a given cell (the terminal stream or the MDBS) is carried on a single DSO trunk, between the MDBS and the MD-IS. Communication between the MDBS and the MD-IS over the DSO trunk follows standard formats such as T1.
Within the CDPD network, digital data is transmitted between the MDBS and the M-ES using Gaussian Minimum Shift Keying (GMSK) modulation. Transmission from the base station to the subscriber station M-ES are continuous. Transmissions from subscriber station M-ES to the MDBS use a burst mode in which subscriber station M-ES only accesses a channel when it has data to send and the channel is not being used by other mobile subscriber stations. This allows a multiple mobile subscriber stations to share a single channel, and for data transmission characterized by intermittent transactions of relatively small amounts of data, thereby greatly reducing the connection time compared to that when sending digital data over conventional circuit-switched cellular modems.
Unlike the signaling schemes used in conventional cellular modems, which have been chosen based on the need to operate within the constraints of the existing voice signaling system, the GMSK modulation technique used for CDPD communication was explicitly selected with the intent of obtaining both very high bit transmission rates and very good error performance in cellular channels. The fact that the choice of modulation was not constrained by a pre-existing signal structure allows CDPD systems to achieve substantially greater instantaneous bit rates at very low received signal levels when compared to those of conventional cellular modems. This means that CDPD communication systems will provide reliable, high speed data transmission in many areas where signal quality is inadequate for good cellular modem performance. Presently the raw (baseband) digital data being transferred across CDPD include electronic mail messages, digital fax data, or digital data representing a network connection such that files may be transferred as if currently connected to a local area network.
The MD-IS handles the routing of packets for all visiting mobile end systems in its serving area. Two services are performed by the MD-IS: a registration service maintaining an information base of each M-ES currently registered in a particular serving location; and a re-address service, decapsulating forwarded packets and routing them to the correct cell. The serving MD-IS also administers authentication, authorization and accounting services for the network support service applications.
A CDPD communication system can operate with dedicated channels set aside from the pool of cellular voice channels and reserved for CDPD use. In the alternative, in a more typical mode of operation, the CDPD communication system can use idle time on channels that may also be used by AMPS communications. In this second case, the mobile data base station may perform xe2x80x9cRF sniffingxe2x80x9d to determine which channels are available and to detect the onset of voice traffic on the channel currently being used for CDPD communication. If an AMPS cellular unit begins transmitting on a channel occupied by a CDPD communication, the CDPD unit ceases transmitting on that channel and switches to another available channel, or if no other channel is available, ceases transmission until a channel becomes available for CDPD use. This is referred to as channel hopping.
Although the CDPD system shares existing AMPS radio frequency channels, as stated above, AMPS calls are given first priority, and they are always able to pre-empt the use of any channel being used by CDPD. However, the cellular service provider may opt to dedicate one or more channels to CDPD usage. In this case, AMPS calls will never attempt to pre-empt the particular channels dedicated to CDPD use.
In a normal operation of the MDBS carrying out channel hopping, the MDBS monitors activity on AMPS channels. The MDBS maintains a list of the status (occupied voice or unused) for each channel available for CDPD use at the cell. The MDBS selects a channel for CDPD use from the unused channels in the list based on a combination of criteria (not specified in the CDPD standard). These could include such considerations as: the likelihood that the channel will be required by the voice system in the near future; the amount of interference present on the channel; the amount of interference that the CDPD communication is likely to cause to other voice users in different cells, or on other sectors; or, other factors. The MDBS transmits a list of all channels available for CDPD use (whether currently occupied by a voice communication or not) to the subscriber stations. The MDBS may execute a channel hop before the channel is pre-empted by AMPS communication if the MDBS determines that another channel is more suitable. In such a case, the MDBS sends a message to the subscriber stations commanding them to change to the specific channel selected, and then the MDBS executes the hop. This sort of hop is much more orderly and efficient since the subscriber stations do not have to search for the next channel.
If the present CDPD channel is pre-empted by AMPS communication, the MDBS selects another channel from those unused by AMPS communications and immediately hops to it without informing the subscriber station. The subscriber station then determines that the CDPD signal is no longer present on the current channel and searches the other channels in the list to determine the channel (if any) to which the CDPD communication has hopped.
The CDPD system has the capability of easily interfacing with the existing AMPS system and sharing some front-end equipment with it. To take advantage of this capability the MDBS must have the capability of physically interfacing with existing AMPS base stations. Consequently, the MDBS should be small, non-obtrusive, and easily accessible without interrupting existing AMPS equipment. The MDBS has to be configured so as to easily be connectable to equipment outside the MDBS normally shared with the AMPS system. This external equipment found in the AMPS base station includes an antenna system; RF power amplifiers (in particular, linear amplifiers can be shared with existing AMPS), RF multicouplers; power splitters; duplexers; and, optional equipment. Since the MDBS shares the environment of the AMPS base station, the MDBS should not constitute a substantial additional burden upon such support systems as environmental control and maintenance. Thus, the MDBS must be compact and flexible, constituting only those elements necessary for carrying out the MDBS functions necessary at that cell site.
As previously described, CDPD systems use packet-switched transmission. However, there are a number of disadvantages to using the packet-switched mode as opposed to the circuit-switched mode. The first disadvantage is economical while the second is a relatively low effective bit rate in the packet-switched mode.
The first disadvantage is based upon the fact that many system carriers will charge for CDPD service on a per-packet basis. On the other hand, circuit-switched service is charged based upon connection time. Thus, under certain conditions, it is economically more advantageous to use the circuit-switched service than the packet-switched service. For example, assuming that a circuit-switched rate is 40 cents per minute (a common rate in the industry) and charges occur for one minute increments, then the least expensive circuit-switched connection will cost 40 cents. Assuming that the CDPD rate is 5 cents per packet (a common rate in the industry), then the CDPD rate is less expensive until more than eight packets worth of data must be sent in a single transaction. Consequently, when large numbers of packets must be sent, it is more economical to use the circuit-switched mode of data transmission rather than the packet-switched mode.
The second disadvantage of using the packet-switched transmission mode resides in the number of data packets from many different subscribers to be found in a data stream. This slows the overall communication of any one subscriber. Further, the extensive overhead necessary to maintain packet-switched transmissions is not required for the circuit-switched mode of transmission. Both of these factors could result in a much lower effective bit rate, and slower communications for individual subscriber stations.
Therefore, the most desirable situation would result from a system that is capable of maintaining a circuit-switched mode of data transmission as reliable as the CDPD data packet-switched mode. Such a system would also have the capability of switching between the packet-switched mode and the circuit-switched data transmission mode at appropriate times. Conventional cellular systems fail to provide these capabilities.
An advantage of the present invention is to facilitate the use of a CDPD Mobile Data Base Station (MDBS) with existing Advanced Mobile Phone Service (AMPS) facilities.
A further advantage of the invention is to facilitate economical data transmission.
An additional advantage of the invention is to decrease transmission time of a data communication over a cellular telephone system.
Still another advantage of the invention is to increase the probability of cellular data transmission completion.
Yet an additional object of the invention is to determine the most favorable mode of cellular data transmission under a particular set of operating conditions.
These and other advantages of the invention are achieved by a communication system arranged to facilitate efficient data transmission where the communication system is constituted by a CDPD. The CDPD portion is arranged to facilitate communication for a plurality of wireless subscriber stations using a plurality of Mobile Data Base Stations (MDBS) controlled by at least one Mobile Data Intermediate Section (MD-IS). The system further includes a continuous CDPD data portion or continuous CDPD portion distributed over a plurality of CDPD MDBS""s, as well as means for a subscriber station to request conversion of a particular data transmission from the CDPD packet-switched portion to the continuous CDPD data portion. The system further includes means for allocating a continuous CDPD data channel for the particular data transmission for which conversion has been requested.
In accord with a second aspect of the invention a method of efficient data transmission in a communication system includes a CDPD packet-switched portion. The CDPD packet-switched portion is arranged to facilitate communication with a plurality of wireless subscriber stations using a plurality of MDBS controlled by a MD-IS. The method of operating the aforementioned system comprises the steps of designating, at the request of the subscriber station using the packet switched mode of CDPD operation, at least one unused channel that has been designated as available for CDPD use as a continuous CDPD channel that is not subject to use by CDPD packet-switched transmissions. In the next step, transferring the particular data transmission requesting subscriber station to the designated continuous CDPD channel and carrying out the particular data transmission in the continuous CDPD mode.
In another aspect of the invention, the aforementioned objects are achieved using a CDPD MDBS comprising a plurality of transceiver cards supporting CDPD communication. At least one of the transceiver cards is allocated to continuous CDPD communication and is not subject to use by CDPD packet-switched mode transmissions. There are also means for assigning data communications to a transmission card allocated for a continuous CDPD data communication.
Yet another aspect of the invention is directed to a portable wireless subscriber station with means for requesting operation in a CDPD packet-switched mode. The subscriber station includes means for requesting that the base station assigns a channel for continuous CDPD transmission, as well as means for selecting between the two means for requesting operation mode.